Metallic effect pigments are platelet-shaped metallic pigments which are distinguished by a particular metallic luster and color effects. The metallic effect pigments are used in paints, varnishes, printing inks, plastics, cosmetics, etc., since they allow production of particular optical effects, especially color effects and luster effects.
The metallic effect pigments are produced conventionally from atomized metal powder by grinding in ball mills. The metal powder required for this operation can be obtained by atomization of molten metal. In the course of the deformative grinding of the atomized metal powder, it is common to add lubricants in order to prevent cold welding of the metal particles to one another. Lubricants commonly used include oleic acid and stearic acid. Oleic acid produces non-leafing properties, stearic acid leafing properties, in the ground metallic effect pigments. Instead of these fatty acids it is of course also possible in each case to use higher or lower homologs as lubricants, such as palmitic acid, for example. Technical-grade fatty acids regularly consist of a mixture of a wide variety of homologous fatty acids, with a saturated fatty acid also always containing certain fractions of unsaturated fatty acids, and vice versa. In the practice of producing metallic effect pigments it is also usual to make use deliberately of mixtures of saturated and unsaturated fatty acids, in other words, for example, a mixture of stearic acid and oleic acid.
Particularly when unsaturated fatty acids are used as lubricating material, the problem arises of the shelf life of metallic effect pigments. The unsaturated fatty acids have a propensity toward polymerization. In the case of the pigments, which are stored mostly in the form of pigment paste or pigment powder, this results in irreversible agglomerations.
According to DE 30 02 175 it is also possible to use dicarboxylic acids as lubricants.
So that the metallic effect pigments have a particularly visually appealing effect in an application medium, as for example in a paint or an ink, it is necessary for the metallic effect pigments to be oriented largely plane-parallel in the application medium, in order to provide directed reflection of the incident light. A random orientation of the metallic effect pigments in the application medium produces an undirected reflection of incident light in all directions, which for a viewer results in a low-grade visual impression.
In an application medium, such as a paint or an ink, for example, metallic effect pigments constitute an alien body or a disruption, which can lead to impaired mechanical properties, on the part of the dried ink or cured paint, for example. These impaired mechanical properties are manifested, for example, in low abrasion resistance, low stability toward environmental effects such as heat, cold, moisture etc. There may also be a splitting of a printing-ink or paint film along the plane in which the metallic effect pigments are oriented, resulting hence in extensive delamination.
WO 99/57204 discloses effect pigments coated with surface modifiers. These surface modifiers have a first reactive functional group which binds to the pigment surface and at least one second reactive functional group which is different from the first functional group and which binds, for example, to the binder system of a paint or of an ink. Although this surface modification increases the mechanical stability of a paint coat or of an ink film, this chemical surface modification has to be carried out in a separate step, which is time-consuming and costly.
DE 24 36 902 discloses an ester composition which comprises polyoxyalkylene glycol groups, monofunctional alcohol groups, and a dibasic acid, the ester having an acid number of less than 25 and a hydroxyl number of less than 25, and comprising 2% to 40% by weight of polyoxyalkylene glycol groups. This ester composition is used in the form of an emulsion with water as a lubricant in the machining of ferrous and nonferrous metals.
WO 2006/070108 A1 discloses a metal pigment composition produced with the metal particles being comminuted in the presence of a fatty acid ester R—COOR, where R is an alkyl radical having 1 to 8 C atoms.
WO 1998/17731 discloses a method for producing a low-dusting or non-dusting metallic effect pigment composition, the metal particles being ground in an aqueous liquid in the presence of corrosion inhibitors and a lubricant. Corrosion inhibitors which can be used include organic phosphorus compounds, examples being phosphate esters of long-chain ethoxylated alcohols.
EP 1 304 210 A1 discloses process auxiliaries for the processing of plastics masses. The process auxiliary comprises partial esters of polycarboxylic acids. The polycarboxylic acids may be dimer acids or trimer acids having 30 to 60 C atoms.
The prior art discloses lubricants or process auxiliaries for the machining or processing of metals or plastics, without any reference to production or use of metallic effect pigments.
The lubricants used to date in the production of metallic effect pigments do not make it possible, following incorporation of these metallic effect pigments into an application medium, to obtain a coating having improved mechanical properties, more particularly a lower splitability of ink layers or paint films, without prior costly and inconvenient surface modification of the metallic effect pigment surface.